7. Describe Drosha and Mirtron pathways for human miRNA biogenesis. (4 points)

(b) How is trp attenuation overridden in E. coli when tryptophan is scarce (缺乏的)? (5 points)

 (a) Present a model to explain attenuation in the trp operon in E.coli. (3 points)

28. Which of the following RNA processing events does NOT take place while eukaryotic gene transcription is underway?
(A) splicing
(B) capping
(C) polyadenylation
(D) cytoplasmic transportation

27. Which of the following descriptions about the polyadenylation signal in mammalian cells is true?
(A) AAUAAA is the most frequent and active sequence.
(B) The polyadenylation signal is followed 23-24 bp later by a UC-rich motif and then by a C-rich motif.
(C) The cleavage/polyadenylation site is within the polyadenylation signal sequence.
(D) The cleavage/polyadenylation site is within the U-rich motif.

26. Which eukaryotic molecule contains a polyA. tail during their biogenesis?
(A) 5S rRNA
(B) tRNA
(C) U6 snRNA
(D) mRNA